1. Field of the Invention
The present invention relates to a closed-end preform that is formed by injection molding of a plastic material, used to form a bottle by stretch blow molding without reheating, and can reduce the weight of the bottle.
2. Description of the Related Art
Examples of the stretch blow molding of plastic materials such as polyethylene terephthalate, polycarbonate, and the like into a bottle include: the cold parison method in which a closed-end preform is injection-molded in a mold, cooled, solidified, and released from the mold, and then reheated to a molding temperature, before it is stretch blow molded into a bottle; and the hot parison method in which a preform in a high-temperature state with its inside not completely cooled and having high internal heat is released from a mold and then stretch blow molded into a bottle while the high-temperature state is maintained (U.S. Pat. No. 5,364,585).
In the hot parison method, the amount of internal heat contained in each part of the preform is proportional to its thickness. Therefore, the greater the thickness is, the larger the amount of internal heat is contained and the easier the stretching can be carried out. As the thickness is reduced by stretching, the surface area increases. Then the temperature in the stretched portion is reduced, and the stretched portion resists stretching. Therefore, stretching then occurs in the adjacent portion having a greater thickness with a higher temperature. The thickness becomes uniform during stretching, and this may be a result of the alternate stretching described above.
Generally, a preform is provided with a draft angle that facilitates release from a mold, and the draft angle is designed such that the thickness of the barrel section of the preform gradually decreases toward the bottom. Therefore, during axial stretching by the extension of a stretching rod, the central portion of the barrel section tends to be stretched first because the central portion has a large amount of internal heat and a stretching force is easily applied thereto, and the bottom section is stretched after some delay.
Since the bottom section is stretched after some delay, it is not sufficiently stretched after the stretching rod reaches the bottom of the mold, and the thickness of the bottom section tends to be greater than that of the barrel section, although it depends on the percent of stretch in the axial (vertical) direction. Therefore, the amount of resin remaining in the bottom section is greater than the amount necessary to form the bottom surface section of a bottle. The barrel section tends to be thin because the amount of stretching increases due to the delay of the stretching of the bottom section. In consideration of this increase, the amount of the resin is slightly increased in some cases.
The barrel section is horizontally stretched and enlarged by the pressure of air that is blown inside the preform from its upper portion with some delay after the extension of the stretching rod. More specifically, the upper portion of the barrel section is first expanded, and the lower portion is then expanded so as to follow the stretched thin portion. The barrel section is thereby stretched horizontally and reduced in thickness, so that the expanded barrel section of the bottle is formed. The bottom section is finally stretched and reduced in thickness, and the bottom surface section of the bottle is thereby formed. However, the bottom surface section of the bottle has a large thickness due to the excess resin.
Attempts have been made to reduce the thickness of the thick bottom surface section of the bottle to reduce its weight. For example, a preform for a bottle having a cylindrical barrel section and a circular bottom section can be designed to have a shape that allows its neck section and barrel section to be easily reduced in thickness. However, it is not easy to reduce the thickness of the bottom section to the extent that contributes to the reduction in weight of the bottom surface section of the bottle. This is because a whitening phenomenon occurs in the central portion of the bottom section due to an increase in the injection pressure and the flow orientation and because the amount of internal heat is reduced to cause difficulty in stretching.
The reason that the bottom surface section of the bottle formed by stretch blow molding of the preform is thicker than the barrel section is insufficient stretching in the axial direction caused by internal temperature difference due to the thickness distribution and by the delay of stretching in the bottom section. Therefore, if the bottom section has a thickness greater than that of the barrel section so that the amount of internal heat in the bottom section is increased, the stretching of the bottom section is facilitated. In this case, the stretching of the bottom section can take place at an earlier time, and the amount of remaining resin due to the delay of stretching is reduced, so that a reduction in thickness may be achieved.
However, to form a preform such that the bottom section is thicker than the barrel section, an undercut must be provided in the bottom section of the mold structure, and the preform cannot be released from the cavity of such a mold and a core mold by drawing. Therefore, new devices must be used to increase the thickness of the bottom section of a preform.
The problem of the undercut due to the increase in thickness of the bottom section of a preform can be solved by the method disclosed in Japanese Patent Application Laid-Open No. Sho 55-146718. More specifically, a preform is formed such that its bottom section has an inverted truncated conical shape. In this shape, the circumferential wall of the bottom section is inclined, and the inner surface of the bottom section faces upward. However, in the inclined circumferential wall of the bottom section formed by bending the lower portion of the barrel section, the thickness of the circumferential wall around the inflection point between the circumferential wall and the barrel section is less than the thickness of the barrel section even though the circumferential wall is bent inwardly.
In addition, in the preform having an inverted truncated conical bottom section, the stretching stress tends to be concentrated on the bent portion between the lower portion of the barrel section and the circumferential wall, and the thickness (internal temperature) of the barrel section differs from that of the circumferential wall of the bottom section. Therefore, stretching first occurs on the barrel side of the bent portion, and the stretching force acting on the circumferential wall is thereby reduced. Accordingly, the bottom section is not stretched as effectively as expected. Moreover, another problem arises in that the thickness of the outer circumference of the bottom edge of the formed bottle can be non-uniform due to the reduction in thickness of the bent portion by stretching.